Screen xerography



Aug. 22, 167 c. SNELLING 3,337,339

SCREEN XEROGRAPHY Filed Oct. 1, 1962 L I 4/ T F/G. Z

" INVENTOR. CHRISTOPHER SNELLING 31%? QQQQ ATTORNEY United States FatentC) 3,337,339 SCREEN XEROGRAPHY Christopher Snelling, Penfield, N.Y.,assignor to Xerox Corporation, Rochester, N.Y., a corporation of NewYork Filed (let. 1, 1962, Ser. No. 227,351 9 Claims. (CI. 96-11) Carlsonin US. Patent 2,297,691 and as further amplified by many related patentsin the field, an electrostatic latent image is formed on aphotoconductive insulating layer and is developed through the depositionthereon of finely divided electroscopic material. The image may be fixedin place or transferred to a sheet of copy paper where it is permanentlyfixed. In most applications the photoconductive insulating layer, whichmay be referred to as a plate regardless of its shape, is first chargedto sensitize it and is then exposed to a light image or other pattern ofactivating electromagnetic radiation which serves to dis sipate chargein radiation-struck areas, thus forming a charge pattern conforming tothe electromagnetic radiation pattern on the plate. This charge patternis then developed or made visible by the deposition on the plate of anelectroscopic or electrostatically attractable, finely divided, coloredmaterial which is referred to in the art as toner. It is the interactionbetween the electrostatic field set up by the retained charge pattern onthe plate and the toner particles which control development of theplate. When utilizing a xerographic plate in a reproduction process onemust consider not only how good the plates are in a photographic sensebut also how the retained electrostatic image on the plate will affectthe development process. Thus, even with a plate that accepts chargewell, holds it for long periods in darkness, and selectively dischargesquickly in response to a light pattern to form a good latentelectrostatic image, certain difficulties may be encountered because ofthe effects of the field or fields set up by this pattern of charge onthe development process.

The fact that charged areas set up relatively strong fringing fields attheir edges or peripheries as compared to the external fields they setup near their centers is basic to an understanding of the interaction ofthese fields with electroscopic developing materials. Actually, thesefringing fields occur not only at the periphery or edge of a chargedarea, but also at any place within a charged area where a high potentialgradient exists. For example, a strong fringing field would exist withina large charged area wherever there are adjacent sections havingrelatively high and relatively low levels of charge. Because of thisfringing field effect xerography has proved to be most useful in thereproduction of line copy subjects, such as printing, line drawings, andthe like, which are represented on the xerographic plate by narrow areasof charge having strong fringing fields at both edges of the lines.Fairly good reproduction of continuous tone originals and originalscontaining large areas of uniform density has been achieved withordinary xerographic plates. However, really good reproduction of thesesubjects has required special techniques to enhance the developmentcapability of those portions of the electrostatic images produced bythese originals which do not have high potential gradients with theirconsequent characteristically strong fringing fields. Thus, additionalapparatus and special techniques such as electrode development asdescribed in US. Patent 2,777,418 to Gundlach, and screen exposure asdescribed in US. Patent 2,598,732 to Walkup, have in many instances,been used to produce really exceptional continuous tone rendition andthe complete copying of very large solid dark areas. This, of course,has required the use of either additional apparatus or additionalprocess steps in the basic xerographic copying system.

Accordingly, it is an object of this invention to define a novel andimproved xerographic plate capable of copying all types of originalsubjects with uniform excellence.

Another object of this invention is to describe a novel method of imagereproduction utilizing the improved xerographic plate of this invention.

A further object of this invention is to define a novel xerographicapparatus of great flexibility which is capable of very high qualityimage reproduction.

The above and still further objects, features, and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed disclosure of specific embodiments of the invention,especially when taken in conjunction with the accompanying drawingswherein:

FIGURE 1 is a cross-sectional view of a xerographic plate according tothis invention.

FIGURE 2 is a side-sectional view of a xerographic copying apparatusaccording to this invention.

Referring now to FIGURE 1 of the drawings, there is illustrated axerographic plate generally designated 11, embodying the concepts ofthis invention.

The present invention contemplates a vastly improved and novelxerographic plate structure which utilizes as its basic mechanism ofoperation an exposure to a uniform light source superimposed on anexposure to the original in the reproduction of images which produce lowpotential gradients on ordinary xerographic plates. The basics of thistype of technique and its advantages are more fully described in U.S.Patent 2,598,732. to Walkup. It is to be noted that, in contrast toWalkup, the structure of the instant invention allows for simultaneousexposure of the plate to the original and a screen pattern without usingtwo projectors.

It should be pointed out that none of the drawings annexed to thisspecification are intended to be scale drawings. Many elements in thedrawings have either been exaggerated or reduced in size so as tofacilitate a clear description of the invention. The plate 11 is made upof a transparent supporting substrate 12 which is sufficiently strong toprovide mechanical support to the remainder of the plate so as to makeit suitable for use in xerographic copying machines. This support membermay be fabricated of almost any transparent material, either conductiveor insulating, and may be selected from such diverse materials as glassand plastics of various types. Depending upon the particular applicationof the plate the support member may be relatively rigid as in the caseof a plate or cylinder of glass or Lucite or may be relatively flexibleas with the case of a plastic web, such as polyethylene or the like.Furthermore, in the event that the remaining plate elements are selfsupporting this supporting substrate may be eliminated entirely from theplate structure. Immediately above the supporting substrate 12 is anoptical screen 13 made up of a number of finely divided, alternating,discrete, opaque, and transparent sections. The screen pattern of opaqueand transparent areas may be a conventional dot pattern or line patternof the type used for out-of-contact screen exposure in the fabricationof halftone plates for newspaper printing. Actually the pattern may beof almost any shape, including round dots, elliptical, lines, and thelike, The spacings of the pattern may also vary so that the pattern isregular, irregular, or random. The pattern may also be varied in sizefrom dot-to-dot or line-to-line. For reasons to be described hereinafterthe adjacent transparent and opaque areas are sharply defined and of thetype referred to in the graphic arts as a hard pattern. Since thepattern in this embodiment is utilized only for optical purposes thepattern may be either conducting or insulating. Thus, almost anycommercially available hard screen pattern from many sources such as theByrum Co. of Columbus, Ohio, and Buckbee-Nears of St. Paul, Minn, may beutilized in the fabrication of this plate. For example, the screenpattern may be made by exposure of the desired pattern to an ordinarysilver halide photographic continuous film negative. If this type of afilm screen is used with a supporting substrate it may be attached tothe substrate with an adhesive or with connectors such as rivets or byother equivalent techniques to produce the desired opaque andtransparent pattern. Immediately above the screen pattern 13 is a verythin transparent conductive layer 14 Which may, for example, befabricated of tin oxide or copper iodide. A copper iodide layer may belaid down on the screen by evaporating a very thin layer of copper onthe screen and then subjecting it to an iodine atmosphere. This may bedone simply by bringing the evaporated copper layer adjacent to a numberof iodine crystals. Copper iodide layers of this type may be made verythin by evaporating copper onto the substrate until the film justbecomes visible and it is believed that they may even reach down intothe sub-micron range. The use of such a thin transparent conductive filmserves to eifectively eliminate any shadow or diffraction eifects whichmight be produced by light passing through this layer from the screen 13and substrate 12 as the light reaches an upper photoconductiveinsulating layer 16, which might have been produced had a thicker layerbeen used.

The formation of this transparent conductive layers of tin oxide andcopper iodide are more fully described in U.S. Patents 2,429,420 toMcMaster, 2,769,778 to Preston, 2,772,190 to Haayman, and 2,756,165 toLyon. As should be clear from a reading of these patents, if a tin oxidelayer is used the screen upon which it is applied must be much more heatresistant.

A test plate of the above type was made starting with a 120 line perinch hard dot pattern on a photographic film known as a Byrum screentint available from the Byrum Co. of Columbus, Ohio. This screen wasprotectively dip coated with a withdrawal rate of 60 inches per minutein a 2 to 1 mixture by volume of a lacquer M-5972 and a thinner EM-998available from the Bee Chemical Co., and dried. The lacquer coated tintwas then placed in a vacuum evaporator about 14 inches above anelectrically heated molybdenum boat containing copper for about /2minute until about 87 mg. of copper was driven off from the boat. Thetint, which was cooled and separated from the boat by a radiation shieldacquired a very thin coating of copper. The tint was then removed fromthe evaporator and placed in an iodine atmosphere until the copperreacted to yield a transparent copper iodide layer with a slight purpletint. This copper iodide coated layer was then overcoated with a layerof amorphous selenium in a second vacuum evaporator and the whole threelayer plate was taped onto a clear Lucite drum with its Byrum tint sidedown after which it was successfully tested in a Xerox 914 oificecopier.

This hard contact pattern and its proximity to the photoconductor alsoallows the use of non point source illumination through the screen suchas ambient room light while still providing very sharp boundariesbetween exposed and unexposed areas. The photoconductive insulatinglayer 16 may be any one of the photoconductive insulating materialsutilized in ordinary xerographic plates and may include such diversematerials as vitreous selenium, any one of many organic photoconductorssuch as anthracene, or inorganics such as certain forms of sulphur,cadmium sulfide, zinc oxide in a film-forming binder, or any othercontinuous film photoconductive insulating material or particulatephotoconductive insulating material or particulate photoconductiveinsulating material in an insulating film-forming binder. It should benoted that the thin transparent conductive layer 14 need not necessarilybe conductive as that term is understood by persons skilled in theelectrical arts. It is only necessary that this layer have sufficientelectrical conductivity for the charging or sensitization of thexerographic plate and to accommodate the release of electrical chargeupon exposure of the plate. Thus, the term conductive as applied to thismember should be read in its broadest sense since its conductivity mustbe high only as compared with the photoconductive insulating layer ofthe plate. Thus, the backing member should have a resistivity lower thanabout 10 ohm-cm. and preferably lower than about 10 ohm-cm. when usedwith a selenium photoconductor.

In an alternative embodiment the optical screen 13 and conductive layer14 may be integrated by utilizing a screen made from a conductivematerial, thus simplifying the construction of the plate and completelyeliminating any possibility whatsoever of diffraction or shadow effectsbetween the screen and the photoconductive layer which might be causedby any material interposed between the screen and the photoconductor.Thus, for example, the screen pattern may be made directly of a materialconductive enough to serve as a xerographic plate base by renderingselected alternating areas across its surface transparent and opaque.

In a third technique the optical screen and conductive base of the plateare combined by utilizing a conductive opaque layer with a number offine, uniformly spaced perforations below the photoconductive layer.Thus a foraminous copper screen made by etching holes in a thinevaporated copper layer may be utilized as a plate base for this novelplate. In this case some of the photoconductor will be deposited in theforarnina of the screen when it is overcoated on the screen but sincethe screen is very thin as compared to the thickness of thephotoconductive layer it does not have a significant eifect on theuniformity of the thickness of the photoconductor. If a plain copperscreen is used with a selenium photoconductor, its surface should beoxidized to provide a good copper-selenium interface barrier layer, thepurpose of which is more fully described in U.S. Patent 2,901,348 toDessauer.

In operation, the plate is first charged with any one of theconventional xerographic plate charging techniques so as to sensitizeit. By way of example, corona charging from a wire filament array asdescribed in U.S. Patent 2,588,699 to Carlson or inductioncharging asdescribed in U.S. Patent 2,934,649 to Walkup may be utilized. Thisprocess is carried out in darkness so that after it is completed theplate has been sensitized with a generally uniform field through thelayer.

During the exposure step of the copying process the front orphotoconductive surface of the plate is exposed to an image by aprojector 17. This image may be of any type, and may include suchdiverse subjects as continuous tone, pictures, line copy, subjectscontaining large solid dark areas, or the like. With an ordinaryxerographic copying process utilizing the conventional xerographic platethe exposure step would normally be followed by development. However,with the novel plate of this invention a second exposure through therear of the plate is carried out at some time between or during platesensitiza tion to the time of development. This exposure is to a uniformillumination as diagrammatically illustrated by arrows 18 in FIGURE 1.This exposure step may be carried out prior to, simultaneous with, orafter charging, subject exposure to the front of the plate at the optionof the operator, the only requirement being that this exposure becarried out prior to the development step. In fact it may also becarried out during development as long as intense enough or far enoughalong at the early stages of development to accomplish the purposes ofthis invention.

This combination of exposure steps serves to modify the electrostaticcharge pattern in such a way that the fields created by charge patternwill develop well even when the original subject was continuous tone orhad large solid areas of relatively uniform density. The rear exposureof the charged photoconductive insulating layer through the screenpattern serves to discharge all areas of the photoconductor directlyopposite transparent areas of the screen. This expo-sure thus serves todivide the plate up into a great multiplicity of very small chargedareas or islands as they will be called for purposes of description.Because of the fact that each of these charged islands is immediatelyadjacent to an area which has been discharged by this rear exposure ahigh potential gradient exists at the outer periphery of each smallcharged area. Because of these high potential gradients each smallcharged island sets up strong electrostatic fringing fields of forcewhich have vertical components above the surface of the plate and arethus very favorable for development with the commonly used xerographicdeveloping materials. Obviously, the number and positioning of thesesmall charged islands will depend directly upon the fineness andtransmission density of the optical screen utilized. Thus, if a coarsescreen of -50 lines per inch is utilized the charged islands remainingafter rear exposure will be relatively large and widely dispersed ascompared with the size and separation of the charged islands resultingfrom the use of a 200 line-per-inch screen. At any event, the plate maybe considered as uniformly charged in the macroscopic sense of the Wordeven after the screen exposure although this uniform charge is made upof the sum of a great plurality of small closely spaced charged islands.

When the subject exposure is superimposed upon the plate it serves tomodulate the charge level of each one of these small charged islandsaccording to the amount of light produced by the exposure from each oneof its different sections. Thus, those small charged islands oppositewhite areas of the subject being copied are substantially completelydischarged by the front exposure of the plate to the subject whilecharged islands on the plate opposite grays in the subject are partiallydischarged and charged islands opposite blacks in the subject retaintheir original charge level. It should be made clear at this point inthe explanation of the invention that the time of the front and rearexposure with respect to each other is immaterial. If rear exposure ismade prior to front subject exposure the charge islands are formed firstand then modulated by the subject exposure from the front of the plate;if both front and rear exposures are made simultaneously the chargedislands are formed by the rear exposure and the level of charge on theislands is modulated by the front exposure simultaneously while if thefront exposure is made prior to the rear exposure the level of chargeacross the whole plate is fi st reduced according to the light receivedfrom the plate from exposure to the subject and this remaining charge isthen divided up into small islands by the subsequent rear exposurethrough the optical screen. Regardless of which sequence is employed theresulting charge pattern on the xerographic plate is identical. In fact,rear exposure may even be made during the beginning of the developmentstep or the end of the charging step.

After exposure the plate bearing an electrostatic image of the typedescribed is developed. Development may be carried out utilizing any oneof the conventional Xerographic developing techniques such as cascade orpowder cloud as described in US. Patent 2,725,304 to Landrigan, amongothers. As described above the effects of the conventional developingtechniques are very favorably infiuenced by the electrostatic fieldsresulting after charging and exposure of the novel plate hereindescribed. Since the charge patterns on the plate are broken up intosmall areas or islands of charge with their consequent fringing fieldseven low levels of charge on the plate corresponding to relatively lightgrays in the original develop well because even these low charge levelsare used in the most effective way possible to attract the oppositelycharged developing particles. Because of this breaking up of the chargepattern and its resultant improvement in the development of light graysthe effective gray scale reproduction of the plate is greatly enhanced.As described more fully above, the development of uniformly chargedareas even including those with high levels of charge is also enhancedsince breaking up the charge pattern into islands produces many smallfringing fields throughout these uniformly charged areas so that theircenters may be fully developed as well as their edges.

In FIGURE 2 there is illustrated an automatic continuous typexerographic copier utilizing a cylindrical plate 19 constructedaccording to this invention. This plate could also be in the form of anendless belt. The plate is made up of three layers including atransparent supporting substrate 21, an optical screen 22, and aphotoconductive insulating layer 23, all of the type described inconnection with FIGURE 1. In this case screen 22 is fabricated of aconductive material; however, if a nonconductive screen were utilized athin transparent conductive layer such as copper iodide or tin oxidewould be included between optical screen 22 and photoconductiveinsulating layer 23. At any event, this transparent conductive layerwould be so thin as to be diflicult of illustration in this figure. Theconductive drum member, whether it be the optical screen itself or aseparate thin transparent layer, is grounded. A light source 24 islocated within drum 19 and is provided with an external switch so thatthe machine operator may turn this light source on or off at will. Ashield may also be provided within the drum so that internal lightsource 24 will only expose a portion of the internal drum peripheryequal to the length of one copy. In this way the internal light may beturned on and off for successive copies. Outside the drum surface thereis a charging unit 26 connected to a source of high potential 27. Thecharging unit 26 contains one or more Wire filaments which are connectedto the potential source and operate on the corona discharge techniquesas described in US. Patents 2,588,699 to Carlson, and 2,777,957 toWalkup. Essentially, this technique consists of spacing a filamentslightly from the surface of a xerographic plate having its conductivebase grounded and applying a high potential to the filament so that acorona discharge occurs between the filament and the plate thus servingto deposit charged particles on the plate surface to raise its level ofelectrostatic charge with respect to ground potential.

The Xerographic plate or drum when in operation is generally rotated ata uniform velocity in the direction indicated by the arrow in FIGURE 2so that after portions of the drum periphery pass the charging unit 26and have been uniformly charged they come beneath a projector 28 orother means for exposing the charged plate to the image to be copied.Subsequent to charging and exposure sections of the drum surface movepast a developing unit generally designated 29. This developing unit isof the cascade type which includes an outer container or cover 31 with atrough at its bottom containing a supply of developing material 32. Thisdeveloping material is picked up from the bottom of container 31 anddumped or cascaded over the drum surface by a number of buckets 33 on anendless driven conveyor belt 34. This development technique which ismore fully described in US. Patent 2,618,552 to Wise and 2,618,551 toWalkup utilizes a two-element developing mixture including finelydivided colored marking particles or toner and grossly larger carrierbeads. The carrier beads serve both to deagglomerate the toner and tocharge it by virtue of the relative positions of the toner and carriermaterial in the triboelectric series. When the carrier beads with tonerparticles clinging to them as cascaded over the drum surface theelectrostatic fields from the charge pattern on the drum pull tonerparticles oif the carrier beads serving to develop the image. Thecarrier beads along with any toner particles not used to develop theimage then fall back into the bottom of container 31 and the developedimage moves around until it comes into contact with a copy web 36 whichis pressed up against the drum surface by two idle rollers 37 so thatthe web moves at the same speed as the periphery of the drum. A transferunit 38 is placed behind the web and spaced slightly from it betweenrollers 37. This unit is similar in nature to the plate chargingmechanism 26, 27, and also operates on the corona discharge principle.This transfer device is connected to a source of high potential of thesame polarity as that employed in the charging device so that itdeposits charge on the back of web 36 which is of the same polarity asthe charge on the drum and is also opposite in polarity to the tonerparticles utilized in developing the drum. This charge on the back ofthe web 36 pulls the toner particles away from the drum by overcomingthe force of attraction between the particles and the charge on thedrub. It should be noted at this point, that many other transfertechniques might be utilized with this invention. For example, a rollerconnected to a high potential source opposite in polarity to the tonerparticles may be placed immediately behind the copy web or the copy webitself may be adhesive to the toner particles. After transfer of thetoner image to web 36 the web moves beneath a fixing unit 39 whichserves to fuse or permanently fix the toner image to web 36. In thiscase a resistant heating type fixer is illustrated, however, othertechniques known in the xerographic arts may also be utilized includingthe subjection of the toner image to a solvent vapor or spraying thetoner image with an overcoating. After fixing the web is rewound on acoil 41 for later use. After passing the transfer station the drumcontinues around and moves beneath a cleaning brush 42 which prepares itfor a new cycle of operation.

When a relatively coarse and inexpensive screen pattern ranging fromabout 80 to 40 lines per inch and below is utilized with the plate ofthis invention a screening eifect is achieved in the final copy becausethe charge pattern, when broken up by the optical screen in back of theplate, forms sufliciently separated portions so that after developmenttheir separation is discernible to the naked eye. Finer screens rangingup to 500 lines per inch and more tend to eliminate this screen effect.The apparatus described in connection with FIGURE 2 is particularlyadvantageous since the light source 24 may be shut off when the systemis being utilized to reproduce line copy subjects and turned on for thereproduction of continuous tone subjects or subjects containing largesolid dark areas so as to eliminate any screen effect in line copy work.When light source 24 is shut off plate 19 is effectively transformedinto an ordinary xerographic plate made up of a photoconductiveinsulating surface layer overlying a conductive backing since theopticalscreen has little or no effect on the charge pattern deposited upon theplate. In this way a relatively coarse screen may be used withoutimparting a screening effect when the apparatus is being utilized toreproduce line copies and even the line copy from a plate incorporatinga relatively fine screen is improved by shutting off the light sourcesince the strong fringing fields at the edges of the lines are notbroken up. As described more fully above, it is unnecessary to break upthe charge patterns representative of line copy on a xerographic platein order to enhance their fringing field effects since these chargepatterns are narrow enough so that their fringing fields are inherentlysuitable for xerographic developer. Thus, by utilizing the FIGURE 2apparatus an operator may secure solid line copy with good definitiontotally devoid of any halftone appearance, or alternatively, by merelyswitching on light source 24 he may greatly Widen the gray scalereproduction of the plate to more perfectly reproduce continuous toneoriginals while simultaneously enabling the full and uniformreproduction of originals having large solid dark areas.

While the specific embodiments shown and described in this specificationand drawings are admirably adapted to fulfill the stated objects of thisinvention, it should be understood that it is not intended to confinethe invention to these disclosed embodiments since it is susceptible ofembodiment in many various forms all coming Within the scope of thefollowing claims.

What is claimed is:

1. A method of xerographic reproduction comprising the steps of:

(a) substantially uniformly electrostatically charging a xerographicplate comprising as an integral member an optical screen having amultiplicity of alternating discrete optically transparent and opaqueareas, a photoconductive insulating layer, a first side of which isadjacent to said optical screen, and including a conductive contact withsaid first side of said photoconductive insulating layer, said contactbeing optically transparent in at least those areas corresponding to thetransparent areas of said optical screen;

(b) exposing the second side of said plate to a light image;

(0) substantially uniformly exposing the first side of saidphotoconductive insulating layer to a light source through said opticalscreen at some time between the later stages of the step of sensitizingsaid plate to the early stages of the subsequent step of developing in amanner such that said two exposures are made to opposite sides of saidphotoconductive insulating layer; and,

(d) developing said photoconductive insulating layer with finely dividedelectroscopic marking material.

2. A method according to claim 1 wherein the two exposures are madesubstantially simultaneously.

3. A method of xerographic reproduction according to claim 1 wherein thescreen-covered side of said photoconductive insulating layer is exposedto ambient light. 4. A xerographic reproducing apparatus comprising: (a)a xerographic plate made up of an endless optical screen having amultiplicity of discrete alternating transparent and opaque areas, aphotoconductive insulating layer, a first side of which is adjacent tosaid optical screen and including a conductive contact with the firstside of said photoconductive insulating layer;

(b) drive means cooperating with said plate to move the plate through apredetermined path;

(c) a light source within the confines of said endless optical screenadapted to uniformly expose the first side of said xerographic plate,

(d) electrostatic charging means positioned along said path and adaptedto uniformly charge the second side of said xerographic plate asportions of the plate pass y;

(e) exposure means next in the path of said plate following saidcharging means to expose said charged photoconductive insulating layerto an image to be reproduced from said second side of saidphotoconductive insulating layer to form a latent electrostatic i g and,

(f) developing means positioned next in the path of said plate adaptedto contact said electrostatic latent image with electroscopic markingmaterial to produce a visible image.

5. A Xerographic apparatus according to claim 4 in which said opticalscreen is in the form of a rigid cylinder.

6. A Xerographic apparatus according to claim 4 in Which said opticalscreen is in the form of a flexible endless belt.

7. A Xerographic apparatus according to claim 4 further including meanslocated after said developing means to transfer said electroscopicmaterial from said Xerographie plate to a copy sheet.

8. A Xerographic apparatus according to claim 4 further including meansto turn said light source Within the confines of said endless opticalscreen on and off whereby low contrast subjects may be moreadvantageously copied by turning said light source on and light contrastsubjects may be more advantageously copied by turning said light sourceoff.

9. A xerographic apparatus according to claim 4 Wherein said conductivecontact consists of an optically transparent, electrically conductivelayer less than about 1 micron thick between said optical screen andsaid photoconductive insulating layer.

References Cited UNITED STATES PATENTS NORMAN G. TORCHIN, PrimaryExaminer.

A. L. LIBERMAN, C. E. VAN HORN,

Assistant Examiners.

1. A METHOD OF XEROGRAPHIC REPRODUCTION COMPRISING THE STEPS OF: (A)SUBSTANTIALLY UNIFORMLY ELECTROSTATICALLY CHARGING A XEROGRAPHIC PLATECOMPRISING AS AN INTEGRAL MEMBER AN OPTICAL SCREEN HAVING A MULTIPLICITYOF ALTERNATING DISCRETE OPTICALLY TRANSPARENT AND OPAQUE AREAS, APHOTOCONDUCTIVE INSULATING LAYER, A FIRST SIDE OF WHICH IS ADJACENT TOSAID OPTICAL SCREEN, AND INCLUDING A CONDUCTIVE CONTACT WITH SAID FIRSTSIDE OF SAID PHOTOCONDUCTIVE INSULATING LAYER, SAID CONTACT BEINGOPTICALLY TRANSPARENT IN AT LEAST THOSE AREAS CORRESPONDING TO THETRANSPARENT AREAS OF SAID OPTICAL SCREEN; (B) EXPOSING THE SECOND SIDEOF SAID PLATE TO A LIGHT IMAGE; (C) SUBSTANTIALLY UNIFORMLY EXPOSING THEFIRST SIDE OF SAID PHOTOCONDUCTIVE INSULATING LAYER TO A LIGHT SOURCETHROUGH SAID OPTICAL SCREEN AT SOME TIME BETWEEN THE LATER STAGE OF THESTEP OF SENSITIZING SAID PLATE TO THE EARLY STAGES OF THE SUBSEQUENTSTEP OF DEVELOPING IN A MANNER SUCH THAT SAID TWO EXPOSURES ARE MADE TOOPPOSITE SIDES OF SAID PHOTOCONDUCTIVE INSULATING LAYER; AND, (D)DEVELOPING SAID PHOTOCONDUCTIVE INSULATING LAYER WITH FINELY DIVIDEDELECTROCOPIC MARKING MATERIAL.